This invention relates in general to color television picture tube degaussing circuits and particularly to picture tube degaussing circuits in color television receivers incorporating switch mode type power supplies.
The need for periodic degaussing or demagnetization of color television picture tubes is well known. Arrangements commonly in use include one or more coils of wire situated closely adjacent to the picture tube and circuit means for producing a high initial amplitude, rapidly decaying, alternating current in the coils to produce a tapered alternating magnetic field for demagnetization. One prior art circuit develops a degaussing field from the large inrush charging current to the electrolytic capacitors in the receiver power supply. A relay is provided for disconnecting the degaussing coils after the inrush current subsides. Another circuit uses a series-connected positive temperature coefficient resistor for tapering the current to the degaussing coils. Still another circuit has a resonating capacitor connected in a tuned circuit arrangement with the degaussing coils, the "ringing" discharge current through the degaussing coils producing a tapered alternating magnetic field for degaussing.
Modern television receivers are increasingly using so-called switch mode power supplies which, though continuously energized, experience very low standby power loss when the load is disconnected. They are therefore very efficient and cost effective. Most of the higher operating voltages required by the receiver are derived from the horizontal output circuit and the switch mode supply has only relatively low voltages available in standby. The low voltage supply to the receiver and the horizontal drive circuit is conventionally switched. Without the horizontal drive circuit being energized, the horizontal output circuit is disabled which, in turn, disables the receiver. Therefore, a simple low voltage switch may be used to control the on-off function of the receiver. Most prior art color television receivers having switch mode power supplies include conventional degaussing circuits as distinct from degaussing circuits using resonating capacitors.
One prior art color receiver with a switch mode power supply does incorporate a resonating capacitor type degaussing circuit. An SCR switch is used to connect the capacitor across the degaussing coils to produce the required tapered current. A reverse polarized diode is connected in parallel with the SCR for conducting current in the opposite direction, as required for degaussing. The SCR switch has a gate electrode which is triggered on and maintained conductive by a transistor switch that is energized from a voltage produced when the receiver's horizontal output circuit is energized (the horizontal output circuit is disabled when the receiver is off). The horizontal output circuit voltage rapidly charges the resonating capacitor immediately upon "turn on" of the receiver through a fairly short time constant charge circuit. Another, longer time constant circuit drives a neon bulb switch which energizes the transistor switch to acitvate the SCR gate. The transistor switch also supplies enabling voltage to the picture tube through a delay circuit which maintains the picture tube non conductive for a short period of time after turn on of the receiver--during which time degaussing occurs. The transistor switch is held conductive while the receiver is on to keep the SCR energized and prevent the resonating capacitor from recharging (and magnetizing the picture tube). Suffice it to say that the circuit is extremely complex and costly.